Image intensifier tubes are used to help an observer see objects under light conditions which would normally preclude vision. The first generation of image intensifier tubes utilizes a fiber optic cathode for receiving light and a fiber optic phosphorus screen for viewing. The screen is an anode which has a cone for focusing the light. As photons of light strike the cathode, the cathode surface generates electrons which are accelerated by a voltage applied across the gap between the cathode and the screen. The cone collects and focuses the electrons onto the screen where a light intensified image is viewed. To prevent the image from becoming too bright for human comfort, various circuits have been used to control brightness or gain.
One type of system which has been used to control gain is an automatic brightness control (ABC). To obtain automatic brightness control in a first generation tube, it was necessary to use an open loop scheme such as by using a soft power supply. When enough current passed through the tube, it caused the voltage to drop, and, therefore, the gain was reduced. The gain was not controlled, however, in an efficient and predictable manner.
Microchannel plates have been previously developed as an alternative to ABC in a second generation of image intensifier tubes. Microchannel plates are able to adjust gain in a closed loop circuit, but unfortunately, microchannel plates are expensive. Thus, a need has arisen for a method and apparatus to provide efficient gain control in an image intensifier tube without the expense of microchannel plates.